SysteMajik Consulting

  • Email Logins for Dovecot and Exim

    While I was cleaning up my Ubuntu Email server configuration, I consolidated my login security. My SMTP server is Exim and my IMAP server is Dovecot.  Mail User Agents (MUAs) use authentication over TLS encrypted connections to access IMAP and SMTP. Both programs had their own password configuration.

    Exim includes Dovecot in its supported authentication mechanisms. This enables one authentication mechanism to be used for both SMTP and IMAP (or POP3). This post also includes configuration details for forced authentication over the Submission port.

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  • Implementing IPv6 Part 2

    We are quickly running out of IPv4 addresses. Are you ready for World IPv6 Day on June 8th, 2011? I have prepared my configuration on OpenWRT and Ubuntu. This includes configuring DNS using bind, email using Exim, and a Squid web proxy.

    Having verified that I could establish IPv6 connectivity, I chose to improve my connectivity. This started with getting a tunnel from Hurricane Electric and updating my configuration. I then updated my bind server and Exim mail server support IPv6 addresses. This posting updates and continues from my post on Implementing IPv6 6to4 on OpenWRT.   Review it for information on creating a tunnel and running radvd on OpenWRT.

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  • Blocking Spam with Exim

    Recent reports indicate that spam is increasing again. I have been using Exim to filter spam for several years. Some recent tuning I have done has decreased the percent of spam that reaches my spam filters. This article provides a discussion of the techniques used and provides implementation examples. Spambots tend to be simple programs that don’t handle slow servers very well. Using a greylist is an effective method of blocking them as they usually don’t retry. My latest changes use delays to cause many spambots to abandon their attempt. Greylisting is used only for poorly configured servers that make it to the Recipient command.

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  • Email Policy

    SysteMajik.com actively discourages Spam and email sent from incorrectly configured servers. A legitimate email from correctly configured servers should have little problem being delivered. We believe we are relatively compliant with  RFC 2505 – Anti-Spam Recommendations for SMTP MTAs and other RFCs mentioned at the end of this document.

    This article covers our policy implementation for incoming and outgoing email.  These policies apply to all email destined to or originating from systemajik.com,  toucantango.com, and other domains for which we may handle email.

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  • Transparent Squid Proxy

    Over the holidays, I had a user experience and attempted browser hijacking. It appeared to have bypassed my squid proxy. My updated configuration now sends all web access via squid. The old firewall rules, that allowed direct access to the Internet, have been replaced with a transparent Squid proxy. This runs on my existing Squid Proxy using another port.

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  • Implementing IPv6 6to4 on OpenWRT

    As the IPv4 addresses begin to run out I finally invested the time to investigate and implement IPV6. As my ISP has not yet announced the availability of IPV6 addresses I chose to implement a 6to4 tunnel. This is simple to implement, and currently well supported. My external firewall is an ASUS wireless router running OpenWRT. As I have a static IP address, my implementation is simpler than is required by a dynamic address. Support for dynamic IPv4 addresses is not covered here, but this configuration should work as long as your address does not change.

    I initially created a 6to4 implementation without a firewall. Then to secure my systems I implemented a firewall using Shorewall6-lite. Until I figured out how to configure the 6tunnel script, I used the command line to bring up the network. This documentation uses the 6tunnel script instead of the manual commands. My configuration does not yet include any IPsec functionality.

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  • Manual networking for KVM

    I found the networking configured by libvirt (KVM) did not allow me to firewall the network as I desired. I use Shorewall for firewalling, and DNSMasq for internal DNS and DHCP.  After a little experimentation, I found that I could configure Ubuntu to create the network. This allows me to get a reliable firewall configuration with a virtual DMZ.

    The virtual hosts are assigned to a bridge, and only have connectivity to other networks as defined in the Shorewall configuration. A single DNSMasq server provides DSN and DHCP services for all virtual servers, as well as the network the server is connected to. The network and firewall configuration remains consistent even as servers are cycled up and down. An additional bridge was created to support virtual servers in the DMZ zone.

    This page has been updated in 2019 to reflect changes in the tools.

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  • Remote Desktops with VNC and RDP

    I find it useful to have a remote desktop to my Ubuntu systems.   On secure connections, I have been using VNC via xinetd.  Connections with xrdp where possible, but it wasn’t launching the desktop for the connection.  For secure terminal connections, I stick with ssh. All these connections have a login at the start of the connection. This is how I do it.

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  • Implementing DKIM with Exim

    This article was updated in February 2014 to reflect changes in policy and reporting options. The earlier ADSP (Author Domain Signing Practices) information has been removed.

    DomainKeys Identified Mail (DKIM) provides a method to confirm the origin of an e-mail. DKIM also provides some protection against tampering. Unlike SPF, this validation applies to the contents of the message when it is signed. Like SPF, the information required for validation is added to DNS.

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  • Signing Return Path Addresses with Exim

    I have been receiving a fair amount of Spam from an e-mail forwarder. They are unwilling to correct their problems. Much of this Spam is in the form of bounce notifications. Attempting to reject other Spam resulted in more notifications. To control this Spam I implemented signed return path addresses. As a side benefit, I am also rejecting bogus notifications sent directly to me.

    Signing my return path allows me to reject faked notification e-mail. The SMTP standard requires that no email sent with a null return path “<>” (aka Envelope Sender) be returned. Its purpose is for allowing for notifications about existing messages. These include notifications such as address unknown, message delivered, and message read. E-mail notifications which are not about a previously sent message can be refused. Signing the return path allowed me to reject such invalid notifications.

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